The invention relates to food tray-containing carts of the general type used by airlines and hospitals to hold food until it is ready to be served. More particularly, the invention relates to the mechanism by which the food trays are kept in a chilled condition from the time they leave the place of preparation until they are removed from the cart. The majority of known cart systems for holding trays use one cart or a portion thereof for holding trays in a chilled atmosphere and another for holding the containers for the hot portion of the meal. The hot meal containers are commonly fully cooked and then merely kept warm in the cart, or else they are partially cooked and kept warm and then brought up to a high temperature before serving. Such systems necessitate the use of a significant amount of labor and space to assemble the hot items to the tray containing the cold items. Furthermore, in the case of a hospital where different patients require different diets, such a system presents a problem of getting the wrong hot item on a patient's tray. Other systems merely insulate the trays so that with the passage of any significant amount of time, the hot food will cool and the cold food will get warm. In order to overcome the aforementioned problems and to greatly improve the palatability of the hot food, there have been a number of systems proposed for keeping all of the food on a tray chilled in a refrigerated atmosphere and then selectively heating the bottom surfaces of the hot items just before serving. Insulating covers on the hot containers help to prevent a loss of heat from the food therein. One such system, which has apertures in the tray bottom through which the hot dishes project into contact with a heater element, is disclosed in Colato et al U.S. Pat. No. 4,005,745. This system includes an enclosure which may contain dry ice and a blower positioned on top of a cart and having short inlet and exhaust conduits extending downwardly through the top wall of the cart to carry chilled air to and from the interior of the cart. Other systems in which the hot containers do not project through the tray bottom are shown in Schulz et al, Reissue Patent No. Re. 30,623 and in Dodd et al, U.S. Pat. No. 4,346,756. In these systems which are especially suited for use in aircraft, a heater shelf carried by the cart is slid under a restrained hot dish as the tray is assembled to the cart. Cooling is shown as being provided through apertures in an end of the cart which are placed in communication with external refrigeration equipment. However, it is broadly noted that other cooling means, including dry ice, could be used as an external source of cooling air, or could be placed in a cart. The latter patent and copending application are assigned to the same assignee as the present application and their disclosures are incorporated by reference herein.
With weight and space both being extremely critical in an aircraft, it is desirable to eliminate the need for separate storage facilities for hot foods as is possible with the cart systems shown in the aforementioned Reissue Patent and Application. However, where chilling is provided by mechanical refrigeration equipment mounted in the aircraft, it is usual to provide a separate, relatively heavy unit for each set of four or five carts which the aircraft can accommodate. These units are not only relatively expensive, but they are quite heavy and each of them must be flown and powered on every flight, even though there are few full tray carts on the aircraft on many of its flights. The mere placement of dry ice within the cart has been found to be insufficient to provide uniform chilling of the closely stacked trays and to overcome the heat loss from the heated containers. The latter loss is not great as the previously chilled containers are heated to serving temperature, but obviously all of the heat produced during any "hold warm" operation made after the containers are brought to serving temperature must be overcome by the chilling medium if the chilled foods are to be maintained at a desired safe storage temperature. A top mounted dry ice cooling unit of the type shown in U.S. Pat. No. 4,005,745 would appear to cause a considerable loss of cart storage area since the cart would have to be much lower to fit in its usual undercounter storage location in an aircraft galley. Also, the inlet and outlet flow conduits which are depicted would appear to be incapable of providing sufficient uniform cooling of all of the trays in an elongated, narrow width aircraft tray cart.